I did some experiments on zindex manipulation at runtime.The general zindex concept with layers and controls is straightforward. Controls can be reordered on the fly.

It gets a bit more tricky when groups are involved. These seem to be on the same Z hierarchy level as controls placed directly onto a layer and zindex can be reordered between these groups or controls directly on the layer.But how can you manipulate zindex within a group? My first experiments did not work.

I put together a quick sample based on your description and was able to reproduce your issue. Reordering controls within a group like this is a totally valid request and the fact that it isn't working is a bug. The issue has been logged and will be addressed. Unfortunately in the meantime, I was unable to find a workaround without removing the group entirely.